Ion beam preparation device for electron microscopy

ABSTRACT

The invention relates to a ion beam preparation device for electron microscopy which is capable of observing the preparation process with the aid of a scanning electron microscope (3) and hence deliberately operate on the sample (4). The device is fitted with a multi-axis sample bench, at least on ion source (1, 2), a scanning electron microscope (3) with electron detectors (7, 9, 8) to image secondary electrons (SE), back-scatter electrons (RE) and transmitted electrons (TE), an electron source as a discharger for isolating samples and a light microscope (5). The ability to observe the etching process in situ permits precise monitoring of the etching stage, e.g. the degree of thickness reduction of the sample, at high resolution and, with the aid of a control device (19), it is possible automatically to terminate the thinning process to precise instructions.

FIELD AND BACKGROUND OF THE INVENTION

The subject of this invention is an ion beam preparation device forprocessing electron microscopy specimens with a vacuum chamber and atleast one ion source for specimen bombardment in a specimen holder by anoble gas ion beam, in particular with argon ions.

For electronmicroscopical observation of specimens the latter must besuitably prepared, e.g. by ion beam etching (D. G. Howit,, Ion Millingof Materials Science Specimens for Electron Microscopy: A Review,Journal of Electron Microscopy Technique 1: 405-414 (1984); A. Garulli,A. Armigliato, M. Vanzi, Preparation of Silicon Specimens forTransmission Electron Microscopy, J. Microsc. Spectrosc. Electron. Vol.10, No. 2, 1985, 135-144).

Ion etching for preparing specimens used in scanning electron microscopy(SEM) and transmission electron microscopy (TEM) is a method that isprincipally used where conventional chemical and electrochemicalprocesses fail or yield only inadequate preparation results. Thisapplies in particular to TEM cross-sectional preparation of material andlayer combinations with strongly selective etching behavior, and tochemically resistant materials. For these cases ion etching hasdeveloped into a routinely practiced method.

Whereas in the beginning cross-section specimens were usually examinedwith transmission electron microscopes with an acceleration voltage of100 kV, preference is now given to medium voltage equipment with 300 kVand field emission sources. This equipment ensures uniformtransmissibility of the cross-section specimens and is able to form beamprobes in the nm range. This establishes the technical preconditions forstructural examination and material analysis (EELS, EDX) in the finestdetails, that is, also on nanostructures.

The development of the nanotechnology, that is, the creation andutilization of structures and dimensions in the submicrometer andnanometer range (e.g. semiconductor component structures), imposessignificantly more demanding requirements on the preparation technique.The necessary etching to the desired thickness of structures withextremely small dimensions requires a significantly better observationpossibility of the specimen during the etching process so that themomentary stadium of the specimen preparation can be accuratelydetermined.

The currently known, conventional ion beam etching systems such as theRES 010 from BAL-TEC, the PIPS model 691 and the Dual Ion Mill fromGatan, as well as the Ion Beam Thinning Unit from Technology LINDA, uselight microscopy with a maximum magnification factor of 100 forobserving the specimens. This is generally inadequate already in thecross-section preparation of simple multilayer systems because themoment at which the etching process is terminated cannot be accuratelydetermined. In the case of specimen etching to the desired thickness ofselected structures, in-situ evaluation of whether or not the structureof interest is located within the thinned down specimen area is entirelyimpossible. This applies already to structures (also samples withperiodic structures) in the μm range! As a consequence an elaborate"trial and error" process is needed in which the specimen must berepeatedly transferred between the etching system and the transmissionelectron microscope or the corresponding specimen holders. This oftenresults in destruction of the specimen.

Another disadvantage of the known ion beam etching systems is the poorcontrollability of the final stage in the etching process. Neither theoptical observability nor the automatic cut-outs known from the knownion beam etching systems allow determination of the exact etchingprocess termination. The sensitivity of the optical and electroniccut-offs used in these equipments is inadequate for switching off theetching process on time. This applies in particular to the ion beampreparation of cross-section specimens. A certain improvement isachieved by the RES 010 from BAL-TEC which uses a special specimenholder with built-in Faraday cup for detecting all charged particles.However, this arrangement severely restricts the possibilities of thespecimen holder because the specimen cannot be thinned and observed onthe back.

In the known ion beam preparation devices the specimens are usuallyrotated during the etching process in order to improve the uniformity oferosion. From patent application U.S. Pat, No. 4,128,765 it is knownthat the specimens should not only be rotated but also the incidenceangle of the ion beam should be varied during the etching process basedon a random function. This is achieved by a rigid arrangement of the ionsources and by reciprocating the specimen holder containing the proberelative to the ion beam by a certain angle. After the etching processthe specimen is ready for electronmicroscopical observation.

In addition to the conventional etching technique described above thereis another technique that was initially developed for fault analysis onmicroelectronic circuits. With the aid of a finely focussed (diam. in nmrange) scanning ion beam the specimen can be etched to the desiredthickness and observed also through ion microscopy. This focussed ionbeam technique (FIB) is currently used also for the preparation ofspecimens for scanning electron microscopy (SEM) and transmissionelectron microscopy. For this purpose the specimen areas of interest arepartially cut through etching by means of Ga liquid metal ion sourceswith extremely high ion densities of up to 10 A/cm², either on one side(slope for SEM examination) or on both sides (ribs for TEM examination).In-situ observation of the etching process is performed with thesecondary particles detached by the ion beam (e.g. Hitachi FB 2000, FEIFIB 200, 600 and 800) and lately also with an additional scanningelectron microscope (FEI Dual Beam FIB/SEM workstation). The finelyfocused ion beam and the ability of accurately positioning the specimenand the ion beam allow accurate etching of the specimen to the desiredthickness. However, as ion beam etching is only possible with astationary specimen this results in strongly preferred structures on theetching slope. This is particularly disadvantageous in multilayersystems with strongly selective etching characteristics. The necessarilyhigh ion current density results in strong back-coating of the etchingslope and strong heating of the specimen. TEM specimens can only beproduced as ribs with a thickness of approx. 100 nm which rendershigh-resolution TEM (HRTEM)examination in selected specimen areasimpossible. As the ribs must be held in place by the remaining specimenmaterial, tilting of the specimen in the TEM for exact specimenorientation is possible only within narrow limits due to the shadingeffect. For producing TEM specimens the specimen surface must be coated.This FIB (focused ion beam) technique has become known, for example,from the Japanese patent application JP 6231720 which corresponds toU.S. Pat. No. 5,525,806. A strongly focused ion beam is scanned at anangle of 90° across the substrate to be processed, where cuboid areasare worked out of the substrate, leaving thin ribs that form the area ofinterest for subsequent TEM examination. The arrangement with thescanning ion beam is operated simultaneously as a SIM, that is, as anion microscope, for observing the etching process. After sufficientetching has occurred on both sides of the rib-shaped area of interest,the area of interest is exposed and the ion source can be switched off.The specimen can subsequently be examined with the SEM.

Simultaneous observation of the specimen during the etching process ispossible only with the Dual Beam FEI in SEM mode. In the case of allother equipment the specimen must be moved into an observation positionand can be observed only with the ion microscope. Even if a lower ioncurrent density and acceleration voltage are used, the disadvantage isthat during the observation material is removed from the specimen areaof interest or that bombardment ions are implanted, both of which alterthe original specimen material.

SUMMARY OF THE INVENTION

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure and are entirely based on the German priorityapplication No. 295 07 225.3 filed Apr. 29, 1995.

The purpose of the invention is to eliminate the disadvantages in thecurrent state of the art, and in particular to etch the specimens underaccurate control in an economical manner with few rejects.

According to the invention this task is solved by the device of otherintention.

The device according to the invention for preparing transmission andscanning electron microscopy specimens allows scanning electronmicroscopic observation of the specimen during the etching process inthe SEM and transmission scanning electron microscopy (TSEM) mode andconsequently continuous evaluation of the momentary etching state aswell as accurate optical determination of the etching processtermination and automatic cut-out of the specimen thinning by means ofthe transmitted electrons registered by an electron detector (TEdetector), without restricting the preparation capabilities of thedevice.

The device according to the invention comprises of a vacuum chamber witha pump system, at least one ion source for noble gas ion bombardment ofthe sample in a specimen holder, preferably argon ions, where also morethan one, preferably two ion sources can be used for achieving higheretching speeds and consequently shorter preparation times. The specimenholder allows rotation and tilting of the specimen. Through the specimenrotation a homogeneous erosion of the specimen surface is achievedduring the ion etching process and the ability to tilt the specimenimproves the variation width of the preparation parameters, e.g. theangle of incidence.

On the vacuum chamber the column of a scanning electron microscope ismounted, the longitudinal axis of which is directed toward the specimen.At least a first electron detector comprising, for example, ascintillator, fibre optic and photomultiplier for registering thesecondary electrons (SE detector) is arranged with a certain tiltrelative to the longitudinal axis of the SEM column so that the specimenholder and the detector, even at different tilting angles of thespecimen holder, are always in a position that is favorable forobtaining an adequate electron signal in the SE detector. In contrast tothe present state of the art the specimen can thus be observed with aresolution in the nm range, allowing accurate evaluation of thepreparation process stage which is of fundamental importance to thesystematic preparation of microfine structures (etching to desiredthickness).

A second electron detector, preferably a semiconductor detector forregistering the transmitted electrons (TE detector), is arranged in theaxial direction of the SEM column behind the specimen and allowstransmission scanning electron microscopic (TSEM) imaging of theelectron transparent areas of the specimen to be thinned down. In thisway an in-situ decision with a resolution in the nm range can be made onhow well the specimen is transparent for electrons, whether or not inthe case of specimen etching to the desired thickness the structures ofinterest are located within the electron transparent area of thespecimen, and whether the etching process should be continued orterminated. The structures can be gauged in-situ and consequentlydifferentiated through line width measurement under software control ofthe scanning electron microscope. The possibility of in-situ evaluationof the preparation result and the etching process termination pointimmediately in the ion beam etching device is another significantadvantage over the present state of the art.

For this purpose the TE detector is equipped with a control device thatis coupled to the power supply of the ion source and which can switchoff the ion source when a preset detector current is registered. Thisautomatic cut-out is much more sensitive than the known automaticcut-outs, and interrupts the etching process before, for example, a holeoccurs in the specimen.

The microscope and the electron detectors are preferably protectedagainst contamination by lockable shutters during the etching of thespecimen. Protection of the sensitive electron detector components andthe scanning electron microscope significantly enhances thereproducibility of the specimen observation and the service life betweencleaning.

Because of its mobility in the x, y and z axes the specimen holder canbe centered relative to the axial direction of the microscope, and theworking distance between the microscope and the specimen holder can beadjusted which means that the specimen can be moved from the preparationposition to the optimum working distance of the microscope andsubsequently restored to the preset working position.

The ion sources used in this arrangement supply a rigid, focusednoble-gas ion beam with ion current densities below 100 mA/cm²,preferably below 30 MA/cm², in order to achieve a short etching timecombined with gentle specimen preparation.

Directly below the exit opening of the scanning electron microscope itis possible to install, for example, a third electron detector,preferably a semiconductor detector, for registering the back scatteredelectrons (BSE detector) so that the specimen can be imaged additionaland selectively either in mass contrast or topography contrast.

At least one ion source, preferably two ion sources, can be tiltablyarranged relative to the specimen holder which allows a large variationwidth of the preparation conditions such as the specimen bombardmentangle as well as one-sided and concurrent two-sided specimen etching.

The invention offers the advantage that the SEM column, electrondetectors, ion sources and specimen holders are arranged in such a waythat the ion beam preparation of the specimen can be observed in-situ atany time, independently of the preparation conditions in the device andwithout interrupting the preparation process.

A light microscope connected to the vacuum chamber is used for observingthe adjustment of the ion beam relative to the specimen. Due to therequired accuracy this adjustment can only be performed with a lightmicroscope.

In the SEM observation of isolating specimens during the ion beamprocessing the bombarding ions compensate the negative specimen chargecaused by the electron beam of the SEM. In the ion beam processing ofisolating specimens it is advantageous to compensate the positive chargecaused by the electron beam of the SEM because in this way uninfluencedspecimen preparation and simultaneously interference-free imaging of thespecimen is possible.

When an isolating specimen is examined at the optimum working distanceof the SEM with the ion source switched off, the negative specimencharge can be compensated by means of an electron source that is aimedat the specimen and operates with electrons in the energy range of 300eV to 1500 eV, preferably 400 eV to 1000 eV. Electrons within thisenergy range have a secondary electron yield >1 and therefore create anelectron impoverishment in the specimen which leads to the compensationof its negative charge.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is subsequently explained in more detailed based on adesign example and references to the drawings. These drawings show:

FIG. 1 is a Schematic illustration of the device for ion beampreparation of specimens used in electron microscopy, with a SEMobservation possibility.

FIGS. 2a, 2b and 2c are schematic illustrations of the SEM column,specimen holder in horizontal position, SE detectors, RE detector, TEdetector and shutters.

FIG. 2a during the preparation of the first specimen side the specimencan be observed by means of the SE detector and RE detector.

FIG. 2b for high-resolution observation the specimen is located withinthe optimum working distance of the SEM.

FIG. 2c during the preparation of the second specimen side up to theelectron transparency of the specimen, the specimen can be observed withthe SE detector and BSE detector as well as the TE detector. If thefinal thinning control is performed with the TE detector, a shutter ispositioned in front of the SEM column and the SE detector which protectsthese components against contamination by ion beam etching.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A design example of the device according to the invention for ion beampreparation of specimens used in electron microscopy with scanningelectron microscopic observation possibility is schematicallyillustrated in FIG. 1. The column of the SEM 3 is mountedperpendicularly on vacuum chamber 10 which can be pumped down to anultimate pressure of 10-6 Torr by a pump system 17. SEM column 3protrudes 90 mm into the vacuum chamber and its longitudinal axis 14 isaligned to specimen 4 located in specimen holder 6.

Specimen holder 6 is mounted at a distance of 50 mm from the undersideof SEM column 3, preferably on a 5-axis specimen stage that allowsmobility as follows:

Rotation and oscillating movement of specimen 4

Tilting around the longitudinal axis of the specimen holder by 0 to 360°

X movement ±15 mm

Y movement ±15 mm

z movement ±40 mm (movement direction specimen holder 6/SEM column 3)

Two opposite ion sources 1 and 2, for example saddle field ion sourcesthat work with an acceleration voltage of 1-10 kV at an ion currentdensity of up to 20 mA/cm² and a beam diameter of approx. 0.5 mm, thebeam of which is aimed at specimen 4, are mobile relative to specimenholder 6 where specimen holder 6 and the ion sources 1, 2 are mutuallyswivel mounted around a central axis of rotation 15. The utilization oftwo ion sources is advantageous because this shortens the preparationtime and enhances the variation possibilities for the preparationconditions. Ion sources 1, 2 can be swiveled jointly or individuallyaround the central axis 15, and individually around an additional axisof rotation.

Tilted by an angle of 45° relative to the longitudinal axis of SEMcolumn 3 the light microscope 5 is arranged which is principally usedfor observing the ion beam alignment relative to specimen 4.

A secondary electron detector 7, preferably of the Everhart Thornleytype, is arranged with an offset of 45° relative to light microscope 5and at an angle of 60° relative to the longitudinal axis 14 of the SEM.In this way a favorable secondary electron detection position of SEdetector 7 relative to specimen 4 and consequently the imaging of thespecimen surface is continually possible, independently of the specimenholder 6 tilting angles used in the device.

As shown in FIG. 2a specimen 4 can be observed in-situ with highresolution of a scanning electron microscope at any time during theetching process, independently of the selectable preparation conditionsof the device and without interrupting the etching process. Theadvantage is that the status of the specimen preparation in the zdirection and the accuracy of specimen etching to the desired thicknessin the x, y, plane can be assessed at any time.

In the layout described above the bombardment angles can be varied asfollows:

When specimen holder 6 is tilted by 15° relative to the light microscope5 the bombardment angle of the first ion source 1 can be varied from-20° to 40°, and the bombardment angle of the second ion source 2 from-40° to 15°, where 0° corresponds to parallel ion incidence relative tothe specimen surface.

If only the first ion source 1 is used, the ion incidence angle relativeto specimen 4 can be varied between 0° and 70° by tilting specimenholder 6 by 45° out of the horizontal position. Specimen 4 alwaysremains in a favorable observation position to the SEM.

For specimen surface observation with higher resolution specimen 4 canbe moved via a 40 mm distance path from the preparation position to theoptimum working distance of the scanning electron microscope of 8 to 12mm as shown in FIG. 2b. After the examination it can be restored exactlyto the preset initial position without having to change the arrangementof the ion sources or the tilting angle of the specimen holder.

Directly adjacent to SEM column 3 there is an electron source 11 aimedat specimen 4 that supplies electrons with energies of 400 eV to 1000 eVand is used for discharging isolating specimens during the SEMobservation at the optimum working distance.

Below the specimen there is a second electron detector 8 that can beshifted in the x, y direction, preferably a semiconductor detector forregistering the transmitted electrons where said detector is used in thefinal thinning process of the second specimen side as shown in FIG. 2and is coupled to power supply 18 of the ion source via a control device19, for example, a computer control. When specimen 4 become electrontransparent this can be observed on the monitor of the scanning electronmicroscope either through light or dark field imaging in TSEM mode, andthe ion sources are switched off with the aid of the registered detectorcurrent in accordance with a set point. In contrast to the knownsolutions this automatic cut-out is highly sensitive and eliminatesunwanted puncturing of the probe.

Subsequently the high-resolution TSEM mode is used to evaluate whetherthe structures of interest are located within the electron transparentarea of specimen 4 and whether or not specimen 4 needs to be furtherthinned or the etching process should be terminated. The structures canbe gauged in-situ and thus differentiated by means of line widthmeasurement via the software of the scanning electron microscope.

An BSE detector 9 in the form of the semiconductor quadrant detector isarranged directly below the exit opening 16 of the scanning electronmicroscope so that the specimen surface can optionally be imaged in masscontrast or topography contrast.

The device consequently allows optimum control of the etching process atall times which is particularly significant in systematic thinning ofselected structures (specimen etching to the desired thickness) so thatthe ion etching process can be terminated at the correct moment.

Another advantage of the device according to the invention is that basedon the high variation width of the bombardment angle and the differentpositioning possibilities of the specimen holder 6, specimens can beproduced with high precision not only for transmission electronmicroscopy (laterally thinned specimens and cross-section specimens) butalso for scanning electron microscopy (e.g. etch slope-cuttings).

During the etching process SEM column 3 and electron detectors 7, 8, 9can be protected against contamination from sputtered sample material bymeans of shutters 12, 13 that can be swung into the path, where shutter12 for the SEM column functions as an electron beam opening so that thefinal stage of the thinning process can still be continuously controlledby means of TE detector 8. In the following two examples of specimensprepared with said arrangement shall be introduced:

Example 1 relates to lateral preparations of a semiconductor componentstructure. The specimen has been mechanically thinned from the substrateside to the ion etching starting thickness of 35 μm. The ion etching wassubsequently performed on the substrate side with two ion sources undera bombardment angle of 6° relative to the specimen surface and anacceleration voltage of 9.8 kV under SEM control until electrontransparentness of the specimen was achieved.

The second example relates to the cross-sectional preparation of acontact hole structure of a multilayer circuit system. The initialthickness of the specimen, achieved by mechanical preparation, was 35μm. The specimen was subsequently thinned down to a predefined depthfrom the first side by means of an ion source under a bombardment angleof 4° and an acceleration voltage of 8 kV with oscillating specimenmovement under SEM control. The ion beam preparation of the secondspecimen side up to the electron transparency of the specimen wasperformed in the same way. The final stage of the thinning process ofboth specimens was successfully controlled with the aid of the TEdetector. As soon as the specimens were electron transparent a picturewith the structures of interest appeared on the monitor of the SEM andthe etching process was terminated on time.

SUMMARY

The invention relates to an ion beam preparation device for electronmicroscopy that can observe the preparation process with the aid of ascanning electron microscope (3) and is consequently able tosystematically process the specimen (4). The device is equipped with amulti-axis specimen stage, at least one ion source (1, 2), a scanningelectron microscope (3) with electron detectors (7, 9, 8) for monitoringsecondary electrons (SE), back scatter electrons (BSE) and transmittedelectrons (TE), an electron source (11) as a discharge device forisolating specimens, as well as a light microscope (5).

The ability to observe the etching process in-situ allows accuratecontrol of the etching state, e.g. the degree of specimen thinning, withhigh resolution, and with the aid of a control device (19) the thinningprocess can be automatically terminated based on a setpoint.

I claim:
 1. Ion beam preparation device for processing specimens (4)used in electron microscopy, comprising a vacuum chamber (10) and atleast one ion source (1, 2) for bombarding a specimen (4) in a specimenholder (6) with noble gas ions for etching the specimen, means forallowing stationary ion beam impingement on the specimen (4) at apredefined angle, means for rotating and for tilting the specimen holder(6) into a fixed position, the vacuum chamber (10) having an SEM column(3) with an axis (14) aimed at the specimen (4), a first scanningelectron microscope (SEM) detector (7) with means for imaging thespecimen surface during operation of the ion source for bombarding thespecimen, and a second SEM detector (8) with means for imaging thespecimen in TSEM mode by registering electrons transmitted through thespecimen during operation of the ion source for bombarding the specimen,the first and second detectors imaging the specimen continuously duringthe etching with noble gas ions.
 2. Device according to claim 1 wherethe second detector (8) is arranged behind specimen (4) along the axis(14) of the electron microscope (3).
 3. Device according to claim 2where the second detector (8) is connected to a control device (19)which is coupled to the ion source (1, 2) in such a way that the ionsource (1, 2) is switched off when a preset detector current is attainedbased on visibly predefined structure elements in the SEM image inaccordance with a degree of specimen etching.
 4. Device according toclaim 1 where a lockable shutter (12) is arranged in front of amicroscope opening (16) of microscope (3) and at least in front of thefirst detector (7).
 5. Device according to claim 1 where specimen holder(6) centers the specimen (4) on the axis (14) of a microscope and theworking distance between specimen (4) and microscope (3) is adjustable.6. Device according to claim 1 where the specimen holder (6) is mobilein five axes.
 7. Device according to claim 1 where the ion currentdensity on specimen (4) is not greater than 100 mA/cm².
 8. Deviceaccording to claim 1 where a third scanning electron microscope (SEM)detector (9) is provided as a back scatter electron detector arrangeddirectly around the scanning electron microscope exit opening (16). 9.Device according to claim 1 including means so that at least one ionsource (1, 2) can be swiveled relative to specimen (4).
 10. Deviceaccording to claim 1 where on vacuum chamber (10) a light microscope (5)is arranged which allows observation of the ion beam alignment relativeto the specimen (4).
 11. Device according to claim 1 where the devicecomprises an electron source (11) that is aimed at specimen (4) wheresaid electron source (11) comprises means for creating electronenergies, for example, of 300 electron Volt to 1500 electron Volt andpreferably 400 electron Volt to 1000 electron Volt.